1. Field of the Invention
The present invention relates to an apparatus for filtering liquids, especially for recovering fibres from so-called white water produced in the paper making industry.
2. Description of the Prior Art
A common type of filtering apparatuses for filtering liquids comprises at least one annular hollow filter disc with side walls covered with filter material adapted to be at least partly immersed in a body of liquid to be filtered and arranged substantially transverse to a horizontal axis extending centrally through the filter disc. There is means for rotating the annular hollow disc about said horizontal axis. Shaft wall means is connected to the filter disc and forms a circular cylindrical chamber, which extends centrally through the hollow filter disc coaxial with said axis and which communicates with the interior of the hollow filter disc. There is means for creating a pressure difference between said body of liquid to be filtered and the interior of the hollow filter disc, such that a filtrate of the liquid is forced through the filter material into the hollow filter disc and passed therefrom into said cylindrical chamber. Filtrate discharge means is provided for discharging said filtrate from the cylindrical chamber, and spray nozzle means is provided for cleaning the filter material.
This type of filtering apparatus is of a simple and inexpensive design. However, a drawback to this apparatus is the poor purity of the filtrate when filtering fibre pulp suspensions, which makes the apparatus unsuitable for fibre recovery. As the filter disc is rotated in a body of white water containing fibres, a relatively large flow of cloudy filtrate is passed through the filter material just after the latter has been cleaned by the spray nozzle means. During the further rotation of the filter material, a mat of fibres is built up on the filter material. This mat of fibres constitutes in itself a tight filter medium, with the result that a relatively small flow of clear filtrate is passed through the part of the filter material which is covered with such a mat of fibres. However, said large flow of cloudy filtrate and said small flow of clear filtrate are mixed in the cylindrical chamber resulting in a poor purity of the filtrate leaving the filtering apparatus.
Another type of filtering apparatus known as "vacuum filtering apparatus" includes filter discs with circumferentially spaced filtrate chambers, which are connected to axial filtrate discharge pipes. During rotation of the filter discs, the filtrate discharge pipes are connected in sequence to two droplegs, with the result that the filtrate is divided into a cloudy filtrate and a clear filtrate. However, if a great number of filter discs is used, such as twenty discs, the rotational speed of the filter discs must be low, in order to prevent too much mixing of the cloudy and clear filtrates. This is so because cloudy filtrate created in the filter discs farthest from the droplegs takes some time to flow through the axial discharge pipes to the droplegs. During this time, the filter discs are rotated a bit further, with the result that the cloudy filtrate from said discs farthest from the droplegs is mixed with clear filtrate from the discs close to the droplegs. Thus, when using such a vacuum filtering apparatus for cleaning white water from fibres, the operational capacity of the apparatus is low because of the necessity to keep an extremely low rotational speed. In addition, a vacuum filtering apparatus is very expensive compared to the type of filtering apparatus which is operated without such droplegs.
U.S. Pat. No. 4,123,363 discloses a vacuum filtering apparatus, in which the above-described axial filtrate discharge pipes are replaced by two axial filtrate discharge channels, which are formed by a stationary partition wall extending axially through a hollow shaft, on which the filter discs are mounted. The two filtrate discharge channels are connected to two droplegs, respectively. There is also a third axial channel, which communicates with the atmosphere via the upper parts of the filter discs, whereby mats of fibres created on the filter material can be removed from said upper parts of the filter discs by means of liquid jets. During rotation of the filter discs, cloudy filtrate is discharged through the first filtrate channel and clear filtrate is discharged through the second filtrate channel, as seen in the rotational direction of the filter discs. In this manner, cloudy and clear filtrate cannot be mixed, independently of the rotational speed of the filter discs.
A problem with this known vacuum filtering apparatus of U.s. Pat. No. 4,123,363 is that communication has to be prevented between said third channel, in which atmospheric pressure prevails, and said two filtrate discharge channels, in which normally negative pressures of about 4 MWG (meter water gauge) prevail, in order to maintain the subpressures in the latter. This requires air-tight seals between the stationary partition wall and the rotating hollow shaft. To obtain acceptable air-tight seals, the clearance between the partition wall and the hollow shaft should not exceed 0.1 mm. However, in practice it would be almost impossible, at least within reasonable costs, to provide such a small clearance along the entire length of the hollow shaft. In ordinary vacuum filter apparatuses, the length of the shaft which carries the filter discs can be up to seven meters. Even if the sealing requirements could be fulfilled, the seals would be worn down in a short time by clumps of small fibres jammed between the partition wall and the hollow shaft.
As to the knowledge of the inventor of the present invention, the apparatus according to U.S. Pat. No. 4,123,363 has not been commercialized, presumably because of the above-related sealing problem.